Power is difficult to produce at millimeter wave frequencies due to the low power output of transistors and the losses incurred by traditional power combiners at these frequencies. Free space combining, also called “quasi-optical” combining, eliminates the latter problem by allowing electromagnetic energy to combine in free space.
Quasi-optical arrays can provide high power by combining the outputs of many (e.g. thousands) of elements. Reflection amplifier arrays are a convenient way to produce power quasi-optically. The reflection amplifier arrays typically have orthogonally polarized input and output antennas in order to reduce mutual coupling between amplifier inputs and outputs. It is desirable to couple inputs and outputs together solely through a partial reflector in order to control the amplitude and phase delay of the coupled energy. Too much “parasitic” coupling between input and output alters the phases of the oscillators, causing decreased combining efficiency and potentially loss of synchronization.
Quasi-optical sources (oscillators) have been developed for millimeter wave power, and consist of a number of individual oscillators that are coupled together so that they mutually synchronize in phase and the radiation from all the elements combines coherently, typically in a (more or less) gaussian mode in front of the oscillator array. A number of different methods exist to realize the coupling network, from printed circuit transmission lines to partial reflectors. The key is to provide strong coupling between elements to ensure in-phase oscillation.
Many embodiments of oscillator arrays utilize “grid” amplifiers in a resonant cavity formed by a ground plane and a partial reflector. In this type of array the grid amplifiers have equal input and output polarizations so that polarization conversion at the partial reflector is not necessary. The drawback with this type of array is that it is difficult to optimize the efficiency since the grid amplifiers themselves are generally not impedance matched and driven under optimal conditions.
Most embodiments in the literature describe arrays that are “transmissive” and not reflective. See for example, J. W. Mink, “Quasi-optical power combining of solid state millimeter wave sources,” IEEE Trans. Microwave Theory Tech., vol. MTT-34, pp. 273-279, Feb. 1986 and Z. B. Popovic, M. Kim, and D. B. Rutledge, “Grid oscillators,” Int. J. Infrared Millimeter Waves, vol. 9, no. 7, pp. 647-654, 1988. This is primarily due to ease of measurements for the transmissive arrays—reflect array performance is difficult to measure since both the source and the load are collocated. However, reflect arrays have the very important advantage of being able to be directly bonded to a heat sink. This is very important for large arrays at millimeter wave frequencies, where efficiency drops considerably and the number of devices per unit area is high.
According to the present disclosure, embodiments of structures are described that collimate both the reflected and transmitted energy, and couples all of the reflected power into the orthogonal polarization, as required by the reflection amplifier array.